A successful, euthyroid outcome after radioiodine therapy in hyperthyroid cats ranges from 83% to 95%. Thyroid volume has been reported as one of the factors influencing radioiodine therapy outcome in man and cats. The goal of this study was to describe the most reliable and practically applicable formula to determine thyroid volume using scintigraphy. The volume of each thyroid lobe of 32 hyperthyroid cats was determined by ultrasound and scintigraphy. The ultrasonographically-determined volume (ellipsoid formula) for each thyroid lobe was compared with the scintigraphic volume that was calculated using eight different formulas: F1 [(π/6) × L × H × W], F2 [(π /2) × L × W(2)], F3 [0.33 × (area cm(2))(3/2)], F4 [1.08 × (π /6) × L × W(2)], F5 (area × H), F6 (0.27 × area × L), F7 (π × L × W(2)) and F8 [π × (4/3) × W(3)]. F1, F3, F4 and F6 did not differ statistically from the volumes measured on ultrasound, while F2, F5, F7 and F8 did. Subjective shape assessment of the thyroid lobes, assigned as cylindrical or spherical, and the use of corresponding formulas, did not appear to be useful.